In paper machines, as in board machines, the headbox plays a significant part in the successful formation of the web. In the headbox, a turbulence generator is also used to create a powerful turbulence in the fibre suspension. After the turbulence generator, there is a slice chamber in the headbox, which is bounded by an upper and a lower lip. The size and particularly the shape of the gap of the slice chamber determine how evenly the fibre suspension is sprayed onto the wire. In addition, the lower lip forms a flow surface, which is made as flat as possible and is often also polished. In other words, the aim is to achieve precise flatness and a small amount of surface roughness, in order to create an undisturbed spray. In practice, however, the upper and lower lips wear. In addition, uneven thermal expansion and deformations caused by accidents lead to changes in the flatness of the lower lip, which disturbs the web formation.
Making various measurements in order to determine the condition of the headbox and particularly the lower lip is a known procedure. The need for repair measures can then be decided on the basis of the results. At present, known measurements are based on a tachymeter or similar levelling device, which is set at the side of the headbox when its side walls are removed. In the measurement, a measuring rod, at which the tachymeter is aimed, is set on the flat surface of the lower lip. The measuring rod is moved in the machine direction and cross direction of the paper machine. The results of the measurement are entered in a record, in which there are data on the measurement object and its dimensions. Correspondingly, an angle gauge connected to a straightedge is used to determine the angle of flow of the headbox.
When measuring in the known manner, the side walls of the headbox must be first be removed. In addition, errors are easily made when placing the measuring rod and when using the tachymeter. Recording also takes a great deal of time as there are tens and even hundreds of points being measured. The accuracy of the measuring rod in the measurement of flatness is at most satisfactory while together with operating errors the final result of the flatness of the lower lip may be erroneous. In practice, the measurement is also slow and at least two trained operators are required to make it.